My sister in law left her belongings in my garage, I have advised her to remove them but she will not take her stuff, how can I get a court order for her stuff to be removed? Thanks!

Normally you don't, you find out the storage laws of your area, you will have to give her written notice to get them, and the date they will be disposed of, according to the storage laws in your state.

Another option, put them into a commercial storage building, pay for it for one month, give her the key and the info, after that tell her it is up to her